The first senolytic agents, senescent cell-eliminating drugs Dasatinib, Quercetin, Fisetin, and Navitoclax were discovered through a hypothesis-driven approach [2À5]: Senescent cells remain viable by inactivating senescence cell anti-apoptotic pathways (SCAPs) including ephrins, PI3K isoforms, p53/p21CIP1, HIF-1a, plasminogen activated inhibitors 1 and 2, and Bcl-2 family members [2]. Therefore, the first senolytics were developed to target SCAP signalling. The work of Le et al. in EBioMedicine [6] is based on the previously reported involvement of the FOXO4-p53 complex [1], a key regulator of SCAPs. FOXO4 is elevated in senescent cells and essential for maintaining viability of senescent cells by sequestering p53 in the nucleus [1]. Baar et al.[1] developed a peptide named FOXO4-DRI to disrupt the FOXO4-p53 interaction by binding to p53 and inducing p53/p21CIP1-dependent apoptosis in senescent cells (Fig. 1). In this study, Le et al.[6] take a different approach by disrupting the FOXO4-p53 interaction with FOXO4 blocking peptides, which enable release and activation of p53. Using an atomistic model to simulate the FOXO4-p53 interaction, Le et al. identify the CR3 domain on FOXO4. A series of peptides were then designed to target high-affinity binding to the CR3 domain. Le et al.[6] further test these putative senolytic peptides in a wide range of senescent cell types. ES2 peptides show the most potent senolytic activity among others and are 3-7 times more effective than the previously reported peptide FOXO4-DRI in both in-vitro and in-vivo studies. Le et al.[6] highlight the safety and efficacy of ES2 in combination with the chemotherapeutic Dabrafenib as a onetwo punch therapy in preclinical mouse models of melanoma. Chemotherapeutics can induce senescence in both cancer and noncancer cells, which promote cancer recurrence and accelerated aging phenotypes [7]. Eliminating therapy-induced senescent cells (TIS) might be an effective strategy to prevent tumour relapse and reduce the long-lasting effects of the disease [8]. Given the heterogeneity of senescent cancer cells in tumour regions and TIS in normal tissues at sites distal to a tumour, the selection and design of senolytics as therapeutics warrant extensive evaluation. Le et al.[6] show compelling evidence for efficacy of senolytics in multiple xenograft cancer models. Notably, the combined therapies of ES2 and a Braf inhibitor result in greater elimination of both cancer and senescent cells as well as significant improvement in survival compared to chemotherapy alone. One caveat to the observations of Le et al. is that senescent cancer cells may not achieve a state of permanent growth arrest and may acquire stemness through mutations leading to drug resistance and ultimately metastasis [9]. Thus, interrogating the long-term effects of senolytics in cancer relapse in treated mice would provide more insight into the true translational potential of this senolytic peptide.

It is worth noting that senescent cells are heterogeneous and vary in SCAPs among tissues of different cell lineages. Therefore, questions to be resolved include understanding which types of senescent cells are responsible for cancer relapse and post-chemotherapy sequelae; and which types of senescent cells are targeted by senolytics. Taken together, use of this class of senolytics clinically requires a deeper understanding of the molecular and functional signatures of eliminated senescent cells as well as characterization of the impact of these drugs on healthy cells. Characterizing senescent cells is necessary to target disease-associated senescent cells while sparing immune-modulating senescent cells or senescent …